This invention relates to liquid tight flexible metal conduit fittings as used in the electrical wiring industry. As is well known to those skilled in the electrical wiring art, flexible metal conduit has a wide number of applications. To couple flexible metal conduit to rigid conduit, distribution panels, hubs or any other device, it is customary to use a liquid tight fitting. Depending upon the environment in which flexible metal conduit is used, it may or may not be covered with a watertight jacket. The invention disclosed herein is described as used with liquid tight flexible metal conduit. Flexible metal conduit, as used in this specification, refers to a flexible conduit which is assembled without conductors and installed in location. Subsequently, conductors are pulled through the flexible metal conduit as is conventional with rigid conduit. Flexible metal conduit of this nature is manufactured from a continuous strip of narrow metal which is formed into the flexible metal conduit by winding the strip in spiral fashion with the edges overlapping and joined together to create a flexible conduit. The joints create helical grooves similar to screw threads both internally and externally of the armor. Flexible metal conduit of this nature is widely used and one form is distributed by the O.Z./Gedney Company with the trade name FLEXI-GUARD.
Flexible metal conduit, of the type cited, is usually cut on a plane at right angles to the axis of the flexible metal conduit. A liquid tight fitting for coupling the flexible metal conduit to rigid conduit, electrical boxes, hubs or the like, must be firmly secured to the flexible metal conduit. Prior art fittings used on flexible metal conduit without watertight plastic jackets employed various techniques for securing the fitting to the armor which relied upon pressure and/or engagement with the outside grooves, or threads, formed during the manufacture of the armor. However, when a watertight covering is used on the flexible metal conduit, such techniques prove unsatisfactory and/or damage the watertight jacket.
In order to provide fittings which provide liquid tight seals and provide an adequate pullout resistance without damaging the watertight jacket, a ferrule was developed which constitutes a cylindrical element having an outside diameter slightly smaller than the inside diameter of the flexible metal conduit and with a thread which will mate with the interior helical seam of the armor. The ferrule could be "screwed" into the armor and the fitting anchored to the ferrule. The ferrule may include an outer cylindrical portion, concentric with the inner cylindrical portion, and with the outer cylindrical portion having an inside diameter which is slightly larger than the outside diameter of the liquid tight flexible metal conduit. A ferrule with the outer cylinder will provide increased resistance to a pullout force. Without the outer cylinder to confine the flexible metal conduit, the conduit may stretch and slip over the threaded ferrule. That is, the outer cylindrical portion restrains the conduit from expanding in response to a pullout force. If the ferrule is not fully engaged with the flexible metal conduit, a reduced resistance to pullout will result. Use of a ferrule with the outer cylinder may result in lack of full engagement between the ferrule and the flexible metal conduit because of careless workmanship, tightness and/or a burr for other obstruction. This disadvantage of the cup-shaped ferrule has resulted in failures and field complaints.